1. Field of the Invention
The present invention relates to sterically hindered mono-substituted amino acids and alkali metal salts thereof and their preparation by a one-step reductive condensation of an amino acid or alkali metal salt thereof and a ketone. It also relates to tertiary amino acids and their preparation by reacting the sterically hindered mono-substituted amino acid or alkali metal salt thereof with an aldehyde to produce the corresponding tertiary amino acid alkali metal salt thereof. The sterically hindered mono-substituted amino acids, the tertiary amino acids, alkali metal salt thereof and their mixtures are useful promoters for alkaline salts in "hot pot" acid gas scrubbing processes.
2. Description of Related Patents and Publications
Recently, it was shown in U.S. Pat. No. 4,112,050 that sterically hindered amines are superior to diethanolamine (DEA) as promoters for alkaline salts in the "hot pot" acid gas treating process. U.S. Pat. No. 4,094,957 describes an improvement to the '050 process whereby amino acids, especially sterically hindered amino acids serve to prevent phase separation of the aqueous solution containing sterically hindered amines at high temperatures and low fractional conversions during the acid gas scrubbing process.
One of the preferred sterically hindered amines described in these patents is N-cyclohexyl-1,3-propanediamine. This amine, while providing good performance in the acid gas scrubbing process, has a tendency to form an insoluble cyclic urea, especially in the presence of H.sub.2 S which is often found in industrial acid gases. See, for example, U.S. Pat. Nos. 4,180,548 and 4,183,903. In addition, one of the preferred amino acids described in U.S. Pat. No. 4,094,957 is pipecolinic acid. It provides good results, but is difficult to prepare and rather expensive.
British Pat. No. 1,305,718 describes the use of beta or gamma amino acids as promoters in the "hot pot" acid gas treating process. However, these amino acids are not suitable because the beta-amino acids undergo deamination when heated for some time in aqueous potassium carbonate and the gamma amino acids under the same conditions form insoluble lactams. Also, the alpha-amino acid, N-cyclohexyl glycine, as described in Belgian Pat. No. 767,105, forms an insoluble diketopiperazine when heated in an aqueous solution containing potassium carbonate.
In view of the commercial potential of using the sterically hindered amino compounds as described and claimed in U.S. Pat. Nos. 4,094,957 and 4,112,050, especially the amino acids, there is a need for providing less expensive amino acids for potential use in this new acid gas scrubbing process. Also, there is a need to find a more suitable alternative to N-cyclohexyl-1,3-propanediamine which performs as well, but does not degrade or form insoluble or undesirable by-products.
It has recently been discovered that certain lower aliphatic sterically hindered monosubstituted amino acids derived from glycine or alanine, especially N-secondary butyl glycine, are quite useful as promoters in the "hot pot" acid gas scrubbing process. These amino acids may be used alone, as co-solvents for other hindered amines, such as N-cyclohexyl-1,3-diamine, or in admixture with tertiary amino acids as hereinafter described. These sterically hindered mono-substituted amino acids not only perform well, but do not form undesirable by-products.
N-secondary butyl glycine has the CAS Registry Number of 58695-42-4 and is mentioned as an intermediate in U.S. Pat. Nos. 3,894,036; 3,933,843; 3,939,174 and 4,002,636, as well as the published literature (Kirino et al., Agric. Biol. Chem., 44(1), 31 (1980). Nothing is said in these disclosures about the synthesis of this amino acid.
Existing processes for producing amino acids, particularly for preparing sterically hindered amino acids frequently involve the use of either expensive starting materials, a plurality of synthesis steps, or costly recovery procedures, either of which may result in high costs to the end-user.
The scientific literature contains a few references showing the preparation of salts of N-secondary alkyl glycines or alanines. For example, Wessely et al., Monatsh. Chem., 81, 861 (1950) describe the preparation of the hydrochloride salt of N-secondary butyl glycine starting from secondary butylamine and glycolonitrile. Greco et al., J. Med. Pharm. Chem., 5, 861 (1962), also isolated the hydrochloride of the amino acid, prepared by reacting the secondary butylamine with ethyl bromoacetate and hydrolyzing the resulting ester with an acid.
The scientific and patent literature contains references to the reaction of aldehydes and ketones with primary and secondary amino compounds. For example, Bowman, J. Chem. Soc. 1346 (1950), Tegner et al., Acta Chem. Scand. 15, 1180 (1961) and Ikutani, Bull. Chem. Soc. Japan, 42, 2330 (1969) describe reductive condensations of amino acids with aldehydes. Glycine, however, reacts with linear aldehydes to give N,N-dialkylamino acids. Branched aldehydes give a mixture of mono- and dialkylderivatives. As an example, isobutyraldehyde reacts with glycine to give N-isobutylglycine and N,N-diisobutylglycine. Also, alanine reacts with linear aldehydes to give mono- or dialkylation, depending on the temperature employed. Branched aldehydes, such as isobutyraldehyde, give only mono-substituted amino acids, e.g., N-monoisobutylalanine.
Reductive alkylations of amino compounds with aldehydes and ketones are also generally described in Organic Reactions, "Preparation of Amines by Reductive Alkylation", Vol. 14, pp. 174-255. Some other examples of these reactions include: U.S. Pat. No. 2,000,034 which discloses the preparation of N-isopropyl-p-aminophenol by the catalytic reduction of p-aminophenol dissolved in acetone; Major, J. Am. Chem. Soc., 53, 4373 (1931) which describes the preparation of N,N-dialkylphenylene diamines from p-phenylene diamine and aldehydes or ketones; and Herada et al., Chem. Abs. 85, 46174 (1976) which reports on similar preparations from p-phenylene diamine and higher ketones by catalytic reduction of the intermediate Schiff bases.
Other disclosures of interest include U.S. Pat. No. 4,190,601 which discloses the reductive alkylation of secondary amines with either aliphatic aldehydes or aliphatic ketones, Skita et al., Ann. 453, 190 (1927) which describes the synthesis of N-alkyl amino acids or their derivatives by reductive condensation (e.g., it shows the reaction of pyruvic acid and an amine or amine derivative); Houben-Weyl, vol. 11, part 2, page 327 (Scheibler and Baumgarten, Ber. 55, 1358 (1922)) which describes the reaction of the bisulfite derivative of acetone with the ethyl ester of glycine.